I don't know any espresso machine that has ever had teflon coated internals. The mystery powder is very likely an espresso backflushing powder, you can add a small amount to hot water and see if it froths up like a concentrated detergent. It's fine to use on your machine, just search the site for silvia backflush technique.
Descaler is certainly the wrong thing to use on portafilters and baskets if you want to make them shiny (it's acidic after all and in strong enough concentration will eat into chrome). The espresso detergent is a much better idea. Mix it up with really hot water, let everything soak for a few hours and then rinse and dry - and don't drink the next coffee, tip it out. As to the black particles, if they're coming out of the steam wand as well they'll be fragments of copper oxide that have flaked off the outside of the heating element. Nothing to worry about in the short term, but this is usually a sign that the machine has been run dry at some stage.